1. Field of the Invention
The present invention relates to a thermal transfer image receiving sheet which is used in combination with a thermal transfer sheet. More specifically, the present invention relates to a thermal transfer image receiving sheet having the same texture and appearance as a plain paper, and the method of manufacturing same.
2. Description of the Related Art
Various types of the thermal transfer recording have been used. As one of the above types, there is known a sublimation type transfer recording. In the sublimation type transfer recording, a thermal head, which generates heat in response to recorded signals, transfers a sublimation dye used as a coloring material onto a thermal transfer image receiving sheet to obtain an image. Recently, the sublimation type transfer recording is applied as an information recording means in various fields. Since the sublimation dye is used as a coloring material, the gradation of a printing density can be controlled as required to reproduce a full color image in accordance with the original image in the sublimation type transfer recording. Furthermore, since the image formed of the dye is very clear and excellent in transparency, the reproduction of intermediate colors and the reproduction of gradation in the image is excellent, thus enabling to form a high quality image comparable to a silver photographic image.
A thermal transfer image receiving sheet using an usual paper as the substrate thereof is proposed as one of the thermal transfer image receiving sheets for the sublimation type transfer recording. The printed matter formed from the thermal transfer image receiving sheet using the usual paper has a comparable quality such as gloss of the surface and the thickness thereof to a printed matter obtained by an usual offset printing or gravure printing. The above thermal transfer image receiving sheet using the usual paper as the substrate thereof can be bent, in addition, a bookbinding or filing thereof is possible even if the thermal transfer image receiving sheets are superposed, contrary to the thermal transfer image receiving sheet using a conventional synthetic paper as the substrate thereof, thus enabling to be used in a various manner. Furthermore, since the usual paper is cheaper than the synthetic paper, it is possible to manufacture the thermal transfer image receiving sheet at a lower cost.
In the above thermal transfer image receiving sheet, a layer having a higher cushion property, for example, a foam layer comprising a resin and a foaming agent, is formed between a substrate and a receptor layer in order to make up the cushion property and the thermal insulating property of the substrate. Thus, a cushion property is provided with the thermal transfer image receiving sheet to improve the fitting property between the thermal transfer image receiving sheet and the thermal transfer sheet. Furthermore, an intermediate layer is further disposed between the foam layer and the receptor layer to prevent the foam layer from being collapsed by heating or pressing upon printing.
In a thermal transfer image receiving sheet in which a foam layer, an intermediate layer and a receptor layer are disposed in this order in layers on such a paper substrate, there is a problem in which a deterioration of the printing quality occurs upon printing in a specific condition.
More specifically, when an image having a lower gradation is printed as the first color at first, and then an image having a higher gradation is printed so as to register with the same region printed with the first color to form a mixed coloring image containing the secondary color or the tertiary color, the gradation becomes discontinuous, after the printing with all colors is finished, to produce a decoloring in a pinhole shape on the printed surface. In the above decoloring portion, the image of the first color has a specific density, and the color density printed at the high gradation (i.e., the second color or the third color) becomes lower than a prescribed density.
The above phenomenon occurs, for example, in a case that an image of bluish green color is formed by printing with light yellow as the first color and mazarine blue as the second color, or that an image of bluish purple color is formed by printing with light red as the first color and mazarine blue as the second color.
This phenomenon is more clearly recognized by observation when the image of the first color comprises many different gradations throughout a low to a high densities, and furthermore the density of the first color continuously and gently changes across the printed image. In the case that the image of the first color is printed in the above manner, the color density of the second color or the third color lowers only in the portions where the image of the first color has the specific density, thus whitish linear portions appear in the printed image.
The above mentioned deterioration in the image quality remarkably lowers the quality of the printed matter, thus extremely lowering the product value. From observing the surface of the printed matter with a microscope, the reason of the above phenomenon is considered to be that the receptor layer is fused and adhered to the thermal transfer sheet when the second or third color is printed, and the receptor layer is peeled off the intermediate layer. More specifically, when the gradation value of the first color is within a specific range, it is seemed that both of the adhesive strength between the receptor layer and the intermediate layer, and the release ability of the receptor layer with respect to the thermal transfer sheet vary depending on the heat history of the receptor layer and reach the following relationship:
Adhesive strength between the receptor layer and the intermediate layer&lt;Adhesive strength between the receptor layer and the thermal transfer sheet.